Two enormous "bubbles" discovered towering above and below our Milky Way galaxy

The heart of our Milky Way galaxy is much more active than most people would realize. In fact, astronomers discovered two gigantic “bubbles” extending above and below the galactic center, roughly 50,000 light years in each direction.

Each one stretches tens of thousands of light-years above and below the galactic center, yet they stay hidden from casual stargazers because they glow mainly in gamma rays and X-rays.

These mysterious formations appear to have emerged from the supermassive black hole at the center of the galaxy, Sagittarius A*.

Researchers pieced together evidence that points to an enormous outburst several million years ago, releasing colossal amounts of energy in a short time.

“Fermi” and “eRosita” bubbles

These enormous structures are called the Fermi and eRosita bubbles, named for the telescopes that found them independently in 2010 and 2020.

Since then, researchers have published well over a hundred peer-reviewed papers that specifically investigate the nature, origins, and implications of the Fermi Bubbles.

One study in particular explores how these bubbles arose and pinpoints the event that may have created them.

It suggests that a powerful jet of material began erupting from the Milky Way’s central black hole, Sagittarius A*, about 2.6 million years ago, continuing for roughly 100,000 years.

Mateusz Ruszkowski from the University of Michigan worked on this investigation alongside partners at National Tsing Hua University in Taiwan and the University of Wisconsin.

“Our findings are important in the sense that we need to understand how black holes interact with the galaxies that they are inside, because this interaction allows these black holes to grow in a controlled fashion as opposed to grow uncontrollably,” said U-M astronomer Mateusz Ruszkowski, a co-author of the study.

“If you believe in the model of these Fermi or eRosita bubbles as being driven by supermassive black holes, you can start answering these profound questions.”

Galaxy bubbles and black hole jets

The research indicates that a black hole outflow can blow away surrounding gas and inflate these bubbles rather than letting the black hole grow without limits.

This idea contrasts with an older hypothesis that connected the bubbles to supernova-driven starbursts.

The new results back the black hole scenario instead, helped by observations of a microwave haze and other emissions near the galaxy’s center.

“We not only can rule out the starburst model, but we can also fine tune the parameters that are needed to produce the same images, or something very similar to what’s in the sky, within that supermassive black hole model,” Ruszkowski said.

“We can better constrain certain things, such as how much energy was pumped in, what’s inside these bubbles and how long was the energy injected in order to produce these bubbles.”

Cosmic rays and expanding gas

The eRosita and Fermi bubbles each shine in different wavelengths, but they appear related to the same violent release of energy.

A shock wave triggered by the Fermi bubbles likely expanded the eRosita bubbles to nearly twice their size.

The expanding cosmic rays can be traced, giving astronomers another way to explore what is happening at the core of our galaxy.

“Our simulation is unique in that it takes into account the interaction between the cosmic rays and gas within the Milky Way,” Yang explained.

“The cosmic rays, injected with the jets of the black hole, expand and form the Fermi bubbles that shine in gamma rays.”

The same explosion pushes gas away from the Galactic center and forms a shock wave that is observed as the eRosita bubbles.

Observing the eRosita bubbles allowed them to more accurately constrain the duration of the black hole activity, and better understand the past history of the Milky Way galaxy.

Creating a galaxy bubble timeline

Multiple lines of evidence hint that the bubble-producing event in the Milky Way galaxy lasted a mere fraction of the timescale once proposed by the starburst explanation.

Researchers involved in this work point out that the starburst model usually implies a longer energy injection period, which would produce different bubble sizes and shapes.

“On the other hand, our active black hole model accurately predicts the relative sizes of the eRosita x-ray bubbles and the Fermi gamma ray bubbles, provided the energy injection time is about one percent of that, or one tenth of a million years,” Zweibel noted.

Injecting energy over 10 million years would produce bubbles with a completely different appearance. It’s the opportunity to compare the x-ray and gamma ray bubbles from each telescope which provides the crucial previously missing piece.

Composition of the galaxy bubbles

The discovery of these towering structures close to home allows astronomers to collect more detailed data than they could from faraway galaxies.

These bubbles also highlight the link between cosmic events and everyday life. The processes involved in creating these bubbles influence how galaxies grow and evolve, which affects the formation of the stars and planets we depend on.

The study’s authors performed simulations that incorporate hydrodynamics, gravity, and cosmic ray physics.

Matching these simulations to real observations helps astronomers understand the energy involved and the composition of the bubbles.

The momentum driving and blowing up these massive Fermi and eRosita bubbles comes from kinetic, thermal, and cosmic ray energy.

Space bubbles and the Milky Way galaxy

To sum it all up, these galaxy bubbles, which seem to be making some type of giant space sandwich out of the Milky Way, likely formed when the supermassive black hole at the center, Sagittarius A*, hurled out energy and hot gas.

Instead of running wild, the black hole pushed material away and created these balloon-like shapes that glow in different types of light – gamma rays and X-rays.

Researchers studied these bubbles by modeling how energy flows through the galaxy, giving them clues about how the outflow grew so large and how it shaped the area around our galactic core.

The results suggest that this wasn’t caused by exploding stars but rather by a focused burst of energy, or jets, from the black hole.

Why does any of this matter?

Now, I totally get it – two giant bubbles hovering above and below our Milky Way galaxy, each over 50,000 light-years long, seems almost impossible to comprehend.

But by learning about these massive bubbles and the energy from the black hole that created them, Sagittarius A*, we learn how galaxies grow, how stars and planets form, and how the universe impacts the environment around us.

That knowledge helps scientists see the “big picture” of where we come from and where we might be headed.

The full study was published in the journal Nature Astronomy.

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